The present invention relates to a semiconductor device having a heat dissipating metal layer on the back side thereof and, more particularly to a method of manufacturing the semiconductor device wherein a semiconductor wafer is divided into chips by laser cutting.
A semiconductor device that must have lower thermal resistance such as high-output FET used in microwave communication or the like generally requires reduced thickness of the semiconductor substrate, for example, 50 xcexcm or less. It is also necessary to form a thick heat dissipating metal layer on the back-side of the substrate in order to improve the heat dissipation efficiency, and to prevent the substrate from breaking during handling.
In Using a dicing method to divide such a semiconductor wafer having the dissipating metal layer into semiconductor chips, which is employed for ordinary semiconductor wafers, clogging of the dicing saw may occur due to the heat dissipating metal sticking thereto. In using a wet etching method instead of the dicing method to divide the wafer, separated semiconductor chips tend to be scattered in a solvent bath at the end of the separation process. The chip scattering makes it time-consuming to collect the separated chips and makes a problem of chip breakage during collection.
Kosaki et al. PCT Application No. PCT/JP96/02758 discloses a method of cutting off a semiconductor wafer into semiconductor chips by means of etching and laser cutting. This method will be described below with reference to the accompanying drawings.
FIG. 5A to FIG. 5G are cross sectional views showing a flow sheet for manufacturing a semiconductor device having a heat dissipating metal layer. First, front separation grooves 3 are formed by etching using a resist pattern 2 as a mask on the surface of a GaAs substrate 1 whereon semiconductor elements have been formed (FIG. 5A). Then a first linkage metal layer 4 is formed in the front separation grooves 3 by plating or other methods (FIG. 5B).
Then wax 5 is applied to the front surface of the GaAs substrate 1, which is then bonded onto a support substrate 6 such as a glass plate or a sapphire plate. Then the back side of the GaAs substrate 1 is polished until the thickness of the GaAs substrate 1 is reduced to about 20 to 30 xcexcm (FIG. 5C). The depth of the front separation groove 3 is typically one-half or less of the thickness of the polished substrate.
A resist pattern 7, which has apertures at positions facing bottom surfaces 3b of the front separation grooves 3, is formed on the back side of the GaAs substrate 1. Using the resist pattern 7 as a mask, back side of the GaAs substrate 1 is etched until the bottom surfaces of the linkage metal layer 4 in the front separation grooves is exposed, thereby to form back separation grooves 8 (FIG. 5D). Width of the bottom surfaces 3b of the front separation grooves 3 must be greater than the width of the bottom surfaces 8b of the back separation grooves 8. In case the bottom surfaces 8b are wider than the bottom surfaces 3b, variations in the amount of etching may lead to over etching where the edge of the bottom surface 8c is excessively etched along the first linkage metal layer 4 (FIG. 6).
After removing the resist pattern 7, a plated feeder layer (not shown) is formed over the entire back surface of the GaAs substrate 1. Then a second linkage metal layer 9 is formed by plating in the back separation grooves 8 for reinforcement of the first linkage metal layer 4. This is followed by the formation of a heat dissipating metal layer 10 by electroplating on portions other than that opposing the bottoms of the front separation grooves 3 (FIG. 5E).
Then the GaAs substrate 1 is pulled off the support substrate 6 and an expand film (not shown) is attached to the heat dissipating metal layer 10. The first and the second linkage metal layers 4, 9 are cut by means of a YAG laser or the like from the side of the first linkage metal layer, thereby separating the GaAs substrate 1 into semiconductor chips 30 (FIG. 5F). The linkage metal layers which have been cut off by the laser have a flange-shaped protrusion 11 with a rounded edge 27.
The semiconductor chip thus produced is bonded onto a package 12 by soldering or the like. Then wires 13 are bonded onto bonding pads located on the surface of the GaAs substrate 1. Last, the entire semiconductor chip is sealed with a resin.
According to the method of cutting the semiconductor chips by laser light, there occurs no problem like clogging of the dicing saw as experienced in the dicing method. Further the semiconductor chips which have been cut by the laser light are arranged orderly on the expand film. Thus there is no need to collect the chips, which is required in the case of using the wet etching method.
With the laser cutting method described above, however, top edges of the first linkage metal layer 4 are located in the same plane as the surface of the GaAs substrate 1. Moreover, the edges of the first linkage metal layer 4 may protrude beyond the surface of the GaAs substrate 1, because the metal formation process based on plating technique or the like. This may cause such a problem that the wires 13 and the linkage metal layer 4 are short-circuited during the wire-bonding processes. Thus it is difficult to stabilize the production yield of the semiconductor device. Moreover, when the device is heated, the wire 13 may be deformed and touch the first linkage metal layer due to thermal deformation of the resin around the wire, this results in poor reliability of the semiconductor device.
The first linkage metal layer 4 and the second linkage metal layer 9 have flange-shaped protrusions 11 after being cut by the laser light. Since the flange-shaped protrusions 11 protrude over a significant length compared to the thickness thereof and the length of the protrusions varies significantly, the protrusions 11 tend to bend during handling the semiconductor chips. The bent protrusions 11 may contact the wires and cause failures of the device.
The present invention has been developed to overcome the above-described disadvantages.
It is accordingly an object of the present invention to provide a method of manufacturing semiconductor devices based on a laser cutting system, which is capable of preventing short-circuiting between the wires and the top edges of the linkage metal layer, and preventing the deformation of the linkage metal layers during chip handling.
This and other objects are achieved in accordance with the present invention with a method of manufacturing a semiconductor device comprising the steps of:
(A) forming front separation grooves between the semiconductor devices on a front surface of a semiconductor substrate by etching;
(B) depositing a linkage metal layer in each front separation grooves;
(C) thinning the semiconductor substrate from a back surface of the semiconductor substrate;
(D) forming a heat dissipating metal layer on the back surface of the semiconductor substrate except opposite the front separation grooves;
(E) forming back separation grooves facing bottom surfaces of the front separation grooves on a back surface of the semiconductor substrate by etching to expose the linkage metal layer; and
(F) cutting off the linkage metal layer by a laser beam to separate the semiconductor devices;
wherein at least one of the steps (A) and (E) comprises two or more separate etching steps, wherein bottom surfaces of grooves formed in the first etching step is further etched in the following etching steps.
The front separation groove forming step (A) preferably comprises following steps,
(a) forming a first resist pattern having a first apertures on the semiconductor substrate;
(b) forming first front separation grooves by etching using the first resist pattern as a mask;
(c) forming a second resist pattern covering side surfaces of the first front separation grooves and a front surface of the semiconductor substrate and having apertures at bottoms of the first front separation grooves; and
(d) forming second front separation grooves continuing below the first front separation grooves by etching using the second resist pattern as a mask. In that case, the linkage metal layer deposited in the step (B) is preferably deposited only in the second front separation grooves using the second resist pattern as a mask. Thus, the top edges of the linkage metal layer is located below the surface of the semiconductor substrate, and short circuiting between the linkage metal and the wires connected to the semiconductor substrate is prevented.
The back separation groove forming step (E) also preferably comprises following steps:
(a) forming first back separation grooves on a back surface of the semiconductor substrate by etching to such a depth that the linkage metal layer is not exposed using the heat dissipating metal layer as an etching mask;
(b) forming a protective film covering the side surfaces of the first back separation grooves; and
(c) forming second back separation grooves by etching to such a depth that the linkage metal layer is exposed using the protective film and the heat dissipating metal layer as etching masks. Thus the back separation grooves consisting of the first and the second back separation grooves are formed.
These steps can reduce the variation of the amount of side etching of the back separation grooves. Therefore, the variation in the length of the protrusion of the linkage metal layer after the laser cutting is reduced.
Also it is preferable to set the depth of the front separation grooves to be deeper, not less than a half of the thickness of the thinned semiconductor substrate. This makes it possible to set the bottom sufraces of the front separation grooves to be narrower than the bottom surfaces of the back separation grooves. This increases the effective area of the semiconductor wafer where semiconductor elements can be formed. Also metal drops sputtered from the linkage metal layer during laser cutting is prevented from depositing on the front surface of the semiconductor substrate, since the portion of the linkage metal layer being cut by the laser is kept far below the surface of the semiconductor substrate.
Another embodiment of the present invention is a semiconductor device comprising:
(A) a semiconductor substrate having a front and a back surface and having semiconductor elements formed at the front surface thereof;
(B) a heat dissipating metal layer formed on a back surface of the semiconductor substrate; and
(C) a metal layer having a flange-shaped protrusion, which has formed on a side surface of the semiconductor substrate to link the semiconductor devices with each other and has cut off by a laser beam;
wherein the semiconductor substrate has four side surfaces, each of which is shaped so as to have an outwardly tapered protrusion defined by a pair of upper and lower side faces that extend to converge at a point laterally outwardly of the substrate; one of said upper and lower side faces has stepped configuration.
Preferably the upper side face of the semiconductor substrate has a shoulder portion, which makes stepped configuration of the upper side face, and the metal layer is formed on the upper side face except the shoulder. The top edges of the linkage metal layer is thus located below the surface of the semiconductor substrate, and short circuiting between the linkage metal and the wires connected to the semiconductor substrate is prevented.
Also it is preferable that the upper side face of the semiconductor substrate has a protrusion laterally outwardly protruding therefrom, which makes stepped configuration of the upper side face, and the metal layer is formed on the upper side face except the protrusion thereof. This configuration gives such an advantage that a junction interface between the metal and the semiconductor substrate is hidden from above the chip.
Preferably the lower side face of the side surface of the semiconductor substrate has stepped configuration formed by a first face and a second face; wherein the first face is formed by a first etching and the second face is formed by a second etching.
This makes it possible to reduce the variation in the amount of side etching during etching of the back separation grooves. Thus, the variation in the length of the flange-shaped protrusion of the metal after laser cutting can be reduced.
It is further preferable that the upper side face of the semiconductor substrate is longer and protrudes beyond the lower side face of the semiconductor substrate. This configuration makes it possible to set the portion of the metal being cut off by laser to be further away from the surface of the semiconductor substrate. Consequently, sputter from the metal during laser cutting can be prevented from depositing on the semiconductor substrate surface. Also the effective area of the semiconductor substrate where the semiconductor elements can be formed are increased.